1. Field of the Invention
This invention relates to a fluid actuated rock drill percussion tool or hammer, and more particularly to such a tool having a fluid distributing central control tube having outlet ports in association with fluid passages in the reciprocating piston to distribute the air through the tool.
2. Description of the Prior Art
All percussion hammers or drills generally comprise a cylindrical chamber having at its upper end means for delivering pressurized fluid to the chamber, and projecting from the opposite end, an axially movable earth boring drill bit. A reciprocating piston is disposed therebetween and, in response to fluid pressure, movable from an elevated position within the chamber to a striking position for delivering an impact blow to the bit to fracture rock thereby, and thence back to a raised position to repeat the impact. The pressurized fluid, which for the most part is air or air and a lubricating or dust inhibiting liquid, is finally exhausted from the hammer to flow back up the borehole, carrying with it the entrained crushed rock for cleaning or bailing the hole.
Distribution of the pressurized fluid within the cylinder to alternately pressurize the volume above the piston while exhausting the volume below the piston to cause the downward striking movement and subsequently, pressurizing the volume below the piston while exhausting the volume above it to raise the piston for another cycle without utilizing a flapper valve or the like, is generally accomplished by various structures such as:having, within the chamber, a central axially extending control tube, having porting corresponding to porting in a central axially extending bore in the reciprocating piston and in flow communication with opposite faces of the piston, such as shown for example in U.S. Pat. No. 4,312,412; having fluid passages in the wall of the cylinder with porting corresponding to reduced diameter portions of the piston for delivering pressurized fluid to opposite faces of the piston; or, a combination of fluid passages through the cylinder walls corresponding to reduced diameter portions of the piston and a central, shortened axially extending tube projecting into a central bore in the reciprocating piston, such as shown for example in U.S. Pat. No. 4,084,646.
The advantages of fluid distribution through a central control tube as opposed to through passages in the cylinder walls are:
(1) it permits a larger piston area for a given outer tool diameter in that the cylindrical wall thickness is not required for fluid passages, which results in a higher energy delivered by the piston impact;
(2) it permits the outer cylinder walls to have a longer wear life because the wall thickness is not interrupted by axially extending passages;
(3) due to the larger diameter piston, it permits a shorter piston length for the same weight, which in turn permits a shorter tool length;
(4) it permits relative ease of porting variation by merely changing the outlet ports in the control tube; and
(5) the outer cylinder, being the wear item, is much less expensive to manufacture without air distributing ports or grooves machined therein.
However, fluid distribution through a central control tube has a major disadvantage which is the premature wear or breakage of the tube caused by misalignment of the relatively narrow-walled, axially extending tube which must be in relatively close sliding and sealing proximity to the walls of the central bore of the piston for minimizing leakage of the pressurized fluid from between these facing surfaces. Further, the axial length of the control tube amplifies any angular misalignment caused by the control tube mount.
Prior art arrangements for mounting the control tube, such as shown in the above-identified commonly owned U.S. Pat. No. 4,312,412, show a control tube received in and extending through a mounting collar; however, because of the necessary machining tolerances and their addition effect upon assembly of the multiple parts, it has been found impossible to maintain the control tube concentric with the piston bore and parallel therewith throughout its axial extent. One prior art solution has included seating the control tube on an O-ring that would accommodate misalignment of the depending tube by permitting the control tube, under the influence of the engaging surfaces of the piston bore and the tube, to force the tube into an appropriate position. This results in undue forceful engagement between the piston and the tube resulting in scuffing and early wear.
Heretofore, none of the control tubes having an axial extent which permits air distributing ports in cooperative alignment with air passages in the reciprocating piston, where the required axial length makes proper alignment more critical, have had a positive assembly alignment to maintain virtual concentricity of the control tube within the cylinder and the central bore of the piston.